Power devices are widely used to carry large currents and support high voltages. Since the early 1950's, developers of electronic power systems began to base their power systems on semiconductor devices.
The power bipolar transistor was first developed in the early 1950's and its technology has matured to a high degree. However, despite the attractive power ratings achieved for bipolar transistors, there are several fundamental drawbacks in their operating characteristics. First, since the bipolar transistor is a current controlled device, large base currents are required to maintain the transistor in the on state. Even larger reverse base drive currents are necessary to obtain high speed turnoff. Bipolar transistors are also vulnerable to a second breakdown failure mode under the simultaneous application of high current and high voltage to the device. It is also difficult to parallel bipolar power devices.
The power Field Effect Transistor (FET) was developed to solve the performance limitations of power bipolar transistors. Power FETs are typically variants of the Insulated Gate FET (IGFET), the Metal Insulator Semiconductor FET (MISFET) also commonly referred to as Metal Oxide Semiconductor FET (MOSFET), or the MEtal Semiconductor FET (MESFET). Power Junction FET's (JFET) may also be provided.
In power FET operation, a control signal is applied to the gate electrode which is essentially a bias voltage. No significant steady state current flows during either the off state or on state. Thus, gate drive circuitry is simplified and the cost of the power electronics is reduced. Moreover, because current conduction in the FET occurs through majority carrier transport only, no delays are observed as a result of storage or recombination of minority carriers during turn-off. Thus, FET switching speeds may be orders of magnitude faster than those of bipolar transistors. Power FETs also possess an excellent safe operating area. That is, they can withstand the simultaneous application of high current and voltage for a short duration without undergoing destructive failure due to second breakdown. Power FETs can also be easily paralleled because the forward voltage drop of power FETs increases with increasing temperature.
In view of the above desirable characteristics, many variations of power FETs have been designed. For example, lateral MESFETs have been developed in gallium arsenide technology for microwave applications. These devices have used a recessed gate structure to obtain channel pinchoff and low source resistance. Unfortunately, the breakdown voltage of these devices is less than 100 V.
Monocrystalline silicon carbide has also been widely investigated for power semiconductor devices. As is known to those having skill in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices, and in particular for power semiconductor devices. Silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturation electron drift velocity compared to silicon. These characteristics would allow silicon carbide power devices to operate at higher temperatures, high power levels and with lower specific on-resistance than conventional silicon based power devices. Recessed gate lateral MESFETs have been reported in silicon carbide. See for example articles entitled Silicon Carbide Microwave FETs by Palmour et al., 1993 International Conference on Silicon Carbide and Related Materials, Abstract We A6; and SIC Microwave Power MESFETs by Siram et al., 1993 International Conference on Silicon Carbide and Related Materials, Abstract We A7. Unfortunately, these devices have breakdown voltages of less than about 100 V.